Tag Archives: air resistance

Tony Martin won the UCI World Time Trial Championship in 2011, 2012 and 2013, and is the odds on favorite to win it again this year. He is an incredible bike rider and by all accounts an incredibly nice guy; he once caught and passed David Millar in a time trial and apologized to Millar as he went by. The picture is of Martin time trialing in full aerodynamic mode at the 2013 Criterium du Dauphine which is an eight-day race that Martin won on the basis of the insurmountable lead he built by winning the time trial.

If you want to minimize air resistance and drag as much as possible, you ride like Martin rides in a time trial. Unfortunately, this is beyond the reach of many riders for a number of reasons. First, it’s expensive. Between his bike frame, wheel set, skin suit, helmet, etc., Martin is moving well over $10K worth of gear in the photo. Next, an optimized aerodynamic position is difficult to accomplish. Differences in body mass and shape mean that the position that optimizes both aerodynamics and power output is different for every rider. Finding that position takes hours of iterative testing and refinement in a wind tunnel. In addition, a full aerodynamic tuck is difficult to maintain and usually takes many hours of practice to be able to hold for any length of time. Simply strapping a set of aerobars unto your bike isn’t going to do it. Finally, riding in an optimal aerodynamic position is difficult. The position is uncomfortable and the bike is more difficult to control when you’re stretched out on aerobars.

Although going full aero involves a significant investment of time, effort and money, there are a number of simple things you can do that will reduce air resistance and drag that are relatively easy and are either free or cost the price of a new jersey. Taken individually, none of these things will result in a huge reduction in air resistance but small things add up and they can make a noticeable difference over a long ride.

Get Low

The biggest single thing you can do to reduce air resistance without spending any money is to reduce the surface area on the front of your body that is exposed to the wind as you ride forward. It’s easy to do and you don’t need aerobars to do it. Bend at the waist and lean forward. The lower you get the better off you’ll be but almost any degree of forward lean will reduce the area of your torso that is exposed to the wind.

This is easiest to do if you have road handlebars on your bike. Moving your hands from a position on the flat upper bar on either side of the stem to the brake hoods will bring you down a little bit. Keeping your hands on the hoods and bending your elbows will bring you down more. You can get fairly low with your hands on the brake hoods by bending your elbows to the point where you’re resting your forearms on the handlebars like the rider in the picture. You can usually get even lower by riding with your hands on the drops.

Elbows and Knees

While the exact elbow position for optimal aerodynamic riding depends on the rider’s body size and shape, it’s generally the case that tucking the elbows in is better than bending them out. Many riders, however, tend to bend their elbows out in a variety of circumstances. When you bend at the waist and lean forward to get low, make sure to pull your elbows in rather than bend them out. People also tend to bend their elbows out when they are leaning into a climb, working hard to maintain a strong tempo, gripping the bars tightly, or are just tired. Pay attention to where your elbows are and if they are flaring wide, bring them in.

Knee position is an aerodynamic factor that people sometimes miss because it’s usually not obvious in pictures of riders who are time trialing. Take a look at where your knees are relative to your body while you’re pedaling. If your knees are spread open so that they form a V-shape with your body, your legs are funneling air into your body and increasing the air resistance and drag you have to overcome. Ride with your knees pulled in toward the top tube on your bike. Obviously, you don’t want your knees banging into the top tube but closer to the tube is better than further away. When you do this, don’t accomplish the “knock-kneed” position by flaring your ankles out.

Your Jersey

Wear a tight jersey. If your jersey is loose or baggy, it is catching the wind and increasing air resistance by acting like a sail that is pulling you in the opposite direction. Don’t worry about what you look like. How you look in your exercise clothes may be a big deal for gym bunnies but for riders the big deal is how smart and how well you ride. If you’re overweight, you’re overweight; there are things you can do to change that but wearing a loose jersey isn’t one of them. Don’t worry about it. Ride smart.

Keep your jersey zipped up. Partially unzipping a jersey allows air to circulate around the upper body to the back where it is trapped by the jersey. It’s like strapping a sail facing the wrong direction on your bike. You would think this would be a no-brainer but you see riders all the time from beginners to pros who have expensive aero gear on their bike and who are trying to go fast with their jersey partially unzipped.

The graph discussed in Aerodynamics Part 1 illustrates that air resistance is negligible when you are going slow but it quickly ramps up with velocity until it becomes the overwhelming force you have to overcome to move your bike forward. While all of the tips and techniques offered in this post have a small effect in reducing air resistance they are all easy to do. They are also either cost-free or cost, at most, the price of a new jersey. Small benefits add up and their combination can make the difference between a ride that is exhausting and a ride that is exhilarating.

You’re riding along at a speed that takes some effort. It’s difficult. What makes it hard? What do you have to overcome that is demanding all that effort to maintain your speed? You ramp it up and go faster. Now it’s even harder and it takes even more effort to maintain the faster speed. Why is it harder? What changed?

When you’re riding your bike there are four factors that resist your forward movement. One of them is gravity but gravity only comes into play when you’re going uphill. You have to provide enough power to carry the combined weight of you and your bike upwards against the earth’s gravitational field. You want to fly up the hill but the earth says not so fast.

When you’re not going uphill, gravity is not doing much of anything to hold you back but it still takes effort to move forward and the faster you go, the more effort it takes. You’re fighting against three types of resistance: drive resistance, rolling resistance, and air resistance. They all take effort to overcome but air resistance is, by far, your biggest enemy.

Drivetrain or drive resistance refers to the force you have to apply to overcome the mechanical resistance coming from the moving parts of the bike’s drivetrain. This includes factors like friction from the chain moving through the teeth on your front rings and rear sprockets and the bearings in the bottom bracket of your bike. If you’re pedaling, you’re producing drive resistance. You can reduce drive resistance by keeping your chain clean and well-oiled and by replacing the bearings in your bottom bracket when they wear out.

Another way to eliminate rolling resistance

Rolling resistance refers to the force you have to apply to overcome mechanical resistance coming from the parts on the bike that are not part of the drivetrain that are moving when you’re rolling down the road. This includes things like the friction of your tires on the road and the bearings in your wheel hubs. You can reduce rolling resistance by making sure your tires are not under-inflated and replacing the bearings in your wheel hubs when they wear out.

Air resistance refers to the force you have to apply to displace the air around you as you move forward. When you move forward your body and your bike are moving into a space that was occupied by air. It would be nice if the air saw you coming and got out of your way but it doesn’t work like that. The air resists and has to be pushed out of the way. As we will see, unless you are climbing a steep hill most of the effort you are putting into pedaling your bike is being used to overcome air resistance.

The graph on the left contains a lot of useful information about where your effort is going when you’re pedaling a bike. The graph shows how much power (measured in watts on the ordinate or y-axis) it takes to move a standard racing or road bike forward at different speeds (measured in kilometers per hour on the abscissa or x-axis). The graph assumes an average sized rider, on an average bike moving forward on a perfectly flat surface – which doesn’t describe any of us with perfect accuracy when we’re out riding our bikes. For this reason, the exact numbers shown in the graph aren’t what is important. When applied to you or me at any moment in time on a specific ride the numbers are likely to be a wee bit off. However the general relationships shown in the graph will hold for all of us all of the time.

Three of the functions on the graph show the amount of power needed to overcome rolling resistance, drive resistance (labeled Drivetrain in the graph) and air resistance (labeled Wind). The fourth function (labeled Total) shows how much power is needed to overcome all three types of resistance combined. Read the graph by taking a velocity or speed shown on the x-axis and then seeing how much power must be applied to overcome rolling, drive, air or the total resistance at that velocity by the corresponding point on the y-axis. For example, at 12 kph (kilometers per hour which is about 7.5 miles per hour or mph) drive resistance is so small it barely registers and it takes about 10 watts of power to overcome rolling resistance and another 10 watts of power to overcome air resistance.

Looking at power in terms of watts may not seem very useful if you do not have a power meter on your bike that tells you how many watts you are producing as you pedal. Another way to look at power is to think of it in terms of effort. What the graph shows as an increase in power measured in watts, can be understood as an increase in the effort it takes to ride at a faster speed. In other words, the higher the power requirement, the more effort you have to expend.

What useful information can we get from the graph?

First, note that the functions for rolling, drive and air resistance all increase with velocity. The faster you go, the more effort it takes to overcome each type of resistance. This isn’t very surprising. Everybody knows it’s harder to go faster and the graph just lets us know that it’s harder for three reasons: rolling, drive and air resistance all increase with speed.

Next, consider the shape of the functions. The rolling and drive functions are linear which means they are very close to straight lines. This tells us something useful. The increase in the effort you need to expend to overcome rolling and drive resistance as you go faster remains roughly constant as your speed increases. For example, it takes the same increase in effort to overcome rolling resistance when you go from 8 to 9 mph as it does when you go from 18 to 19 mph. In other words, no matter what your current speed is, the same increase in effort will be enough to overcome the increase in rolling resistance when you go 1 mph faster.

Now take a look at the shape of the function for air resistance. It curves sharply up; it’s not a straight line. This is not good news for the cyclist. It means that equal increases in speed (for example, an increase of 1 mph) demand an ever-larger amount of effort the faster you go. For example, it takes more effort to overcome air resistance when you go from 8 to 9 mph than it does to overcome air resistance when you go from 7 to 8 mph and it takes a lot more effort to go from 18 to 19 mph than it does to go from 8 to 9 mph. In other words, overcoming air resistance gets harder and harder the faster you go. When you get to the speed where the pros ride overcoming air resistance takes enormous increases in effort to produce tiny increases in speed.

Overcoming air resistance gets harder and harder the faster you go. How quickly does the increase in effort needed to overcome air resistance ramp up?

Looking at the effort needed to overcome air resistance as a proportion of the total effort expended can answer this question. At approximately 12 kph (7.5 mph) overcoming air resistance is taking about half of your effort with rolling and drive resistance accounting for the other half. As you go faster, overcoming air resistance demands a larger and larger proportion of your total effort. By the time you get to 25 kph (15.5 mph) approximately 70% of the effort you are putting out is being used to overcome air resistance. When you get to 20 mph (about 32 kph) roughly 85% of your effort is devoted to overcoming air resistance. And it just gets harder after that. According to the UCI (the governing body for professional cycling) the typical average speed for a flat stage in the Tour de France is about 47 kph (29.2 mph). The power demands for overcoming air and total resistance are off the chart for our graph but from the information on the graph we can calculate that over 90% of the effort needed to maintain this speed is used to overcome air resistance.

What would riding a bike be like if you didn’t have to overcome air resistance? Reading from the graph, it takes about 100 watts of power to overcome the combination of rolling, drive and air resistance when you’re going 25 kph (15.5 mph). If you take air resistance out of the equation and imagine the linear functions for rolling and drive resistance extending beyond the 50 kph boundary on the right side of the graph and continuing to increase at roughly the same rate, that same 100 watts of power would be sufficient to move you forward (on a very rough estimate) at a speed of about 125 kph (77.7 mph).

Okay, air resistance is the problem. What’s the solution? There are many ways to reduce air resistance such as riding in a full aero position on the bike which is both difficult and uncomfortable, spending tens of thousands of dollars on a high tech aerodynamic bike and a sleek aerodynamic kit, and some simple and much less demanding riding techniques that can provide small but significant benefits. We’ll examine some of these simple techniques in Aerodynamics Part 2.